Influence of small color differences on the contrast threshold: its application to atmospheric visibility

Abstract

In a laboratory experiment the influence of very small chromaticity differences on the threshold of the human eye was measured. A considerable reduction of the luminance threshold was found when chromaticity differences under their perceivable threshold were presented simultaneously. The results were compared with MacAdam’s formula for combined luminance and chromaticity differences. These laboratory results were applied to the horizontal atmospheric visibility, using some field measurements of the inherent contrast of several natural materials and the spectral reflectance of typical colored paints. The chromaticity difference at visibility distances plays an important role, making some objects visible at distances where the luminance contrast is under its threshold. However, the influence of the chromaticity correction should influence the visibility by no more than 10%. The propagation in the atmosphere breaks the symmetry of the eye’s sensitivity curve, the long-wavelength part of the contrast almost determining the chromaticity difference and thus increasing the visibility of the object seen against the horizon.